Method of heat-treating low temperature tough steel

ABSTRACT

(Heat) In the disclosed heat -treating method of hot-rolled steel plate containing less than 0.20% of C, 0.05 to 0.40% of Si, 0.10 to 5.0% of Mn and 1.50 to 10.0% of Ni ( 1.5% to 10.0% Ni and 0.1% to 5.0% Mn and further at least one element selected from the group consisting of Mo, Cu, Cr, Nb and V, at need and) with the balance being substantially Fe, (comprising subjecting one or more time repeatedly said) the hot-rolled steel plate (to heating) is heated to (and sequent quenching from) a temperature (within) between the (range between) Ac1 and Ac3 transformation points and then cooled, whereafter (, then subjecting) the steel plate is subjected to tempering treatment at a temperature below the Ac1 point but sufficiently high so as to cause precipitation or formation of fine austenite, thereby to improve (largely) toughness at low temperature and to obtain excellent workability.

United States Patent 1191 Aoki et a1.

Nippon Steel Corporation, Tokyo, Japan [22] Filed: Nov. 5, 1973 [21] Appl. No.: 412,625

Related US. Patent Documents [73] Assignee:

[ Reissued Dec. 9, 1975 2,797,162 6/1957 Korczynsky 148/134 2,992,148 7/1961 Yeo et a1. 148/134 3,135,600 6/1964 Hardwick et a1. 148/134 3,249,426 5/1966 Nakamura 148/143 3,266,947 8/1966 Steiner .1 148/12 3,444,011 5/1969 Nagashima et a1. 148/31 Primary Examiner-W. Stallard Attorney, Agent, or Firm-Toren, McGeady and Stanger [57] ABSTRACT Heat 1n the disclosed heat-treating method of hotrolled steel plate containing less than 0.20% of C, 0.05 to 0.40% of Si, 0.10 to 5.0% of Mn and 1.50 to 10.0%

kessue of Ni I: 1.5% to 10.0% Ni and 0.1% to 5.0% Mn and [64] Patent 326192302 further at least one element selected from the group lssued- 1971 consisting of Mo, Cu, Cr, Nb and V, at need and PP 776,421 with the balance being substantially Fe, comprising F'led: 1968 subjecting one or more time repeatedly said the hotrolled steel plate to heating is heated to and se- 52 US. (:1. 148/12 F; 148/124; 148/134; quen, quenching from a temperature [within be- 148/143 tween the I: range between Ac and Ac transforma- [51] Int. Cl. C2111 1/26; C21d 7/14 points and men Cooled, whmafle, the [58] Field of Search 48/12 1341 I43; jecting the steel plate is subjected to tempering treat- 75/123 ment at a temperature below the Ac point but sufficiently high so as to cause precipitation or formation of [56] References Cited fine austenite, thereby to improve largely tough- UNITED STATES PATENTS ness at low temperature and to obtain excellent work- 2,532,117 11/1950 Newell 148/134 y- 2,664,369 12/1953 Kiefer, Jr. 1 148/134 2,679,454 5/1954 Offenhauer 148/134 8 12 Drawmg figures S E Q P '2 3o f i, V 1 1a 1 1 1 .L o l 1, 1 1 2 1 1 g 20 r 1 1 I "15 to l 1- 1 1-, 1

o e 1 1 is 10 c: 1: I o J: o g 5 t: 5 E l E 0 1 1 1 1 1 1 N O 200 300 4 O0 500 600 700 Tampering Temperature (C) Reissued Dec. 9, 1975 Sheet 1of5 Re. 28,645

FIG.

0 o 7 O O 6 ll llllllllllll l O oc O O//. 5 w 0 o 4 O 0 3 O 0 2 O O O 3 2 w 5 0 $59 9: 0 mm, 3 3 EB. 095 820 522 EEN Tempering Temperature (C) Reissued Dec. 9, 1975 Sheet 2 of5 Re. 28,645

FIG]

Reissued Dec. 9, 1975 Sheet 4 of 5 Reissued Dec. 9, 1975 Sheet 5 of5 Re. 28,645

METHOD OF HEAT-TREATING LOW TEMPERATURE TOUGH STEEL Matter enclosed in heavy brackets I appears in the original patent but forms no part of this reissue specification; matter printed in italics indicates the additions made by reissue.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat-treating method for imparting to a low carbon tough steel containing 1.5 to 10.0% by weight ofNi and 0.1 to 5.0% by weight Mn a very high toughness at low temperature, particularly at such an extremely low temperature as l96C, the boiling point of liquid nitrogen, and an excellent workability, particularly low yield ratio at room temperature [to a low carbon tough steel containing 1.5 m 10.0 wt.% Ni and 0.1 to 5.0 wt.% Mn II.

2. Description of the Prior Art It is well known that recently a 9%-Ni steel has been developed which is widely used as a steel material for use at low temperature. This 9%-Ni steel is usually made manufactured into a finished product after having been subjected to hardening and tempering treatments, or to normalizing and tempering treatments, after a solid solution treatment. The thus treated product shows a relatively good toughness at low temperature, but [it does not always follow that a satisfactory results can not always be obtained in case an optimum toughness, superior to the above-mentioned, is required.

As regards [to workability, no particularly excellent result could be obtained according to the experiments made by the inventors of the present invention.

Further, a 6%-Ni series low temperature tough steel has been developed by the inventors of the present invention as a low temperature tough steel which should take the place of the 9%-Ni steel and For this new steel US. Pat. No. 3,388,988 was granted. By subsequent investigations, however, it was discovered that in order to obtain an optimum toughness, a tempering treatment must be carried out under the optimum conditions of a relatively narrow range, which make a knotty point in the operation is cumbersome from an operational point of view. Also as regards the workability, the optimal result could still not still] be secured with the 6%-Ni steel, as [in is the case [of] with the 9%-Ni steel.

Now, as seen from examples of 6%-Ni and 9-Ni steels, in general, with When a usual heat treatment [which is to be] is carried out with 6%-Ni and 9%-Ni steels for obtaining a high toughness, the yield ratio becomes high, but a steel having a high yield ratio is used to be detested by a I not favored by construction designers from the view point of safety. As is explained in the following, the present invention [is to provides a heat-treating method, by which a steel having a high toughness can be obtained in [the a wide range of a tempering temperature range, while keeping the yield ratio low by utilizing a segregation.

SUMMARY OF THE INVENTION The present invention [is to eliminates the abovementloned defects of Ni-containing steels. Therefore,

the primary object of the present invention is to pro vide an excellent tough steel which I: is by far :I ha. significantly improved [in the 1 toughness character istics, by causing an a Ni-containing steel to form fine structure according to a method [mentioned] explained in the following, without causing thereby reduction in a] strength at low temperature.

The second object of the present invention is to im part an excellent workability, particularly a propert of 1 low yield ratio, to [an a Ni-containing tougl steel.

Another object of the present invention is to providt a tempering treatment [of an 1 for a Ni-containin; steel with the optimum conditions I: of in 2 wide [r range.

Still other objects of the present invention will be come manifest by the following description [5 witl reference to the examples and accompanying drawings The present invention, which is to attain the above mentioned objects, provides the method, wherein a hot-rolled steel plate containing 1.5 to 10.0% Ni, 0.1 tr 5.0% Mn, and Si in an amount as required for making steel (preferably 0.05 to 0.4%) and further containing at least one element properly selected from the group consisting of 0.05 to 1.0% Mo (a part or the whole of Mo may be replaced by W), 0.10 to 2.0% Cu 0.10 to 1.50% Cr, less than 0.05% sol, Al, less thar 0.2% Nb, less than 0.2% V, less than 0.1% Zr, less that 0.1% Ti, less than 0.005% B, as the occasion demands and the balance being Fe and unavoidable impurities, is heated to a temperature above the Ac transformatior point and is then cooled with a cooling rate greater than [a] obtained by air cooling [by air] and thereafter is heated to a temperature between the Ac, transformation point and the Ac, transformation point and is then cooled (with a cooling rate greater than [a] obtained by air cooling [by air] and then whereupon the thus treated steel plate is further subjected to a tempering treatment at a temperature below the Ac, transformation point [J or the hot-rolled steel material may be directly heated to and quenched from a temperature between the Ac, transformation point and the Ac, transformation point. and then whereupon the thus treated steel plate is subjected to a tempering treatment at a temperature below the Ac, transformation point. Further, prior tc the heating and cooling to and from a temperature above the Ac, transformation point or to and from a temperature between the Ac, transformation point and the Ac transformation point, a solid solution treatment may be carried out.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing the relation between a tempering temperature and a toughness of a 6%-Ni series steel treated according to the method 01 the present invention, as compared with that of a 6%-Ni series steel treated according to a conventional method, i.e., quenched and tempered.

FIG. 2 is a photograph showing a structure by the observation with an electron microscope of a magnification of 4,500 of a 6%-Ni series hot-rolled steel plate as heated to 800C. (above the Ac, transformation point] and cooled therefrom.

FIG. 3 is a photograph showing a structure by the observation with an electron microscope of a magnification of 4,500 of [in the case where 1 the steel of FIG. 2 [is after having been further heated and sequently 3 oled to and from 670C. FIG. 4 is a photograph showing a structure by the obrvation with an electron microscope of a magnificam of 4,500 of [in the case where the steel plate of G. 3 [is I which has been further subjected to a temring treatment at 600C. FIG. 5 is a photograph showing a structure by the obrvation with an electron microscope of a magnifica- III of 4,500 of [in the case where the steel plate of G. 2 which is directly subjected to a tempering treat- :nt at 600C. FIG. 6 is a diagram showing the relation between a tempering and [a toughness of a 9%-Ni series :el treated according to the method of the present inntion, as compared with that of a 9%-Ni series steel :ated according to a conventional method. FIG. 7 is a photograph showing a structure by the obrvation with an electron microscope of a magnifican of 4,500 of a 9%-Ni series hot-rolled steel plate as ated to 800C. and cooled therefrom. FIG. 8 is a photograph showing a structure by the ob- 'vation with an electron microscope of a magnifican of 4,500 of [in the case where] the steel plate of G. 7 [is] after having been further heated to 670C. d sequently cooled therefrom. FIG. 9 is a photograph showing a structure by the obvation with an electron microscope of a magnifican of4,500 of [in the case where] the steel plate of 3. 8 [is] which is subjected to a tempering treat- :nt at 575C. IG. 10 is a photograph showing a structure by the servation with [must be I an electron microscope a magnification of 4,500 [in the case where] ofthe el plate of FIG. 7 which has been directly heated [to 'ectly 6 to 575C. and sequently cooled therefrom. IG. 11 is a photograph showing a structure by the servation with an electron microscope of a magnifition of 4,500 in the case where ofa 3%-Ni series t-rolled plate which is subjected to the heat-treat- :nt method of the present invention. IG. 12 is a photograph showing a structure by the servation with an electron microscope of a magnifi- :ion of 4,500 [in the case where] ofa 3%-Ni series t-rolled steel plate which is subjected to hardening :i tempering treatments.

DESCRIPTION OF THE PREFERRED EMBODIMENT lhe essential feature of the present invention resides producing a structure consisting of a fine ferrite and npered martensite, wherein a stable ultrafine austenor cementite is precipitated, by subjecting a hotled steel plate containing necessary elements in the iges as above mentioned or a steel plate obtained by rdening or air-cooling the hot-rolled steel plate, to :cial heating and cooling treatments to and from a nperature between the Ac transformation point and Ac transformation point and thereupon subjecting thus treated steel plate to a tempering treatment, ich enables the steel to display an excellent tough- ;s at a very low temperature.

[It was manifested by the The investigations made the inventors of the present invention demonstrated vt a relatively good toughness could be displayed :n at low temperature, when a tough steel containing in a relatively great amount, as a 9%-Ni steel or a -Ni steel, was subjected to hardening and tempering atments. However, it was further discovered that, if

LII

a hot-rolled steel plate having the composition as above mentioned was subjected to a heat treatment specified by the present invention, that is heating to a temperature within a range between the Ac, transformation point and the Ac transformation point and sequent quenching or air-cooling from the said temperature, there could be obtained such an I excellent toughness at low temperature and workability I: as was never seen at any values were obtained which were far superior to those displayed by the conventional Ni-containing steels, such as 9%-Ni steel or 6%Ni steel L] whereb the application of J The inventive steel I: for use& is therefore particularly suitable for use at low temperature [was widely spread]. Further, as such a remarkable improvement in the toughness at low temperature as above mentioned could be obtained without adding new alloy elements, that is, with a steel having the composition in the same or even smaller narrower ranges as the conventional 9%-Ni steel or 6%-Ni steel, a very economical production of an excellent tough steel for use at low temperature was made possible.

In the process of manufacturing the steel of the present invention, I: a 1 melting may be carried out in any well known steel-making furnace such as converter, open-hearth furnace, electric furnace, high frequency furnace and the like. There is no problem [in the point in respect of melting. A molten steel prepared by melting in any of the said furnaces is regulated in its composition by adding alloy elements as required and is made to a hot-rolled steel plate through the steps of ingotting, slabbing and hot-rolling. Then, the thus obtained hot-rolled steel plate is heat-treated as specified by the present invention.

The starting material of] for performing the present invention is a hot-rolled steel plate as above mentioned. In the present invention, the hot-rolled steel plate may be immediately subjected to the special heattreatment of the present invention (a solid solution treatment may be carried out prior to this special heattreatment) or, as another method in another embodiment, the hot-rolled steel plate may be heated at first to a temperature above the Ac transformation point but below the crystal grain coarsening temperature, with or without the solid solution treatment to be carried out prior to this heat-treatment. In this case, it is not desirable to heat the steel plate to a temperature above the crystal grain coarsening temperature, be cause the toughness of the steel will be deteriorated thereby. This heating is followed by a cooling rate greater than [an] obtained by air-cooling, that is, quenching, or by I: an air-cooling. The structure of the steel after this treatment I: becomes is that of a martensite structure or a mixed structure of martensite and bainite (or a mixed structure of martensite, bainite and ferrite according to circumstances).

The steel plate as hotrolled or further subjected to the heat-treatment to a temperature above the AC3 transformation point and sequent quenching or aircooling as above mentioned, is further successively heated to a temperature in the range between the Ac transformation point and the AC3 transformation point and sequently quenched or air-cooled.

The essential feature of the present invention is just this special heat-treatment, that is, a heating to a temperature between the Ac, transformation point and the Ac transformation point and sequent cooling from the said temperature. By this heat-treatment a fine structure of the steel is produced, whereby the toughness as well as the workability of the steel can largely be improved without reducing the strength thereof at low temperature. That is, when the above-mentioned steel plate as hot-rolled or further subjected to the heattreatment to a temperature above the Ac transformation point and sequent quenching or air-cooling, is further successively heated to a temperature in the range between the Ac, transformation point and the Ac, transformation point, a fine austenite, enriched in C, Ni, Mn and N, present as an impurity, is precipitated or formed in an insular form at an old martensite crystal grain boundary, old austenite crystal grainboundary or ferrite subgrain boundary. with the help of an effect of accelerating the I The precipitation or formation of the fine austenite is facilitated by the effect of accelerated diffusion caused by the presence of a great number of dislocation groups in the quenched or air-cooled structure which is a result [ed from of the previous treatment II. and this This austenite is equilibrated with well annealed and fine ferrite having an excellent toughness, resulting in the formation of a mixed structure of [them fine austenite and fine ferrite. In the present invention it is an indispensable condition that the mixed structure of austenite and ferrite is produced by the heating to a temperature between the Ac, transformation point and the Ac, transformation point. The most preferable range in the above-mentioned heating to a temperature between the Ac, transformation point is 620 to 800C. in the case of] ifthe steel having has the composition range as above mentioned. Through the sequent quenching or air-cooling, a mixed structure of ferrite and fine insular martensite can be obtained. The quenching or air-cooling from the above-mentioned temperature range may be carried out once or may be repeated several times. By repeating the said treatment several times the martensite structure is further refined, whereby the toughness of the refined steel is all the more improved. The medium for use in the quenching may be water, oil, mist or any other medium which [can performlcauses a compulsory cooling. There is no particular limitation in the medium to be selected.

The thus heat-treated steel plate is then tempered at a temperature below the Ac, transformation point, preferably in the range of 450 to 600C. By this tempering treatment, austenite is again dispersedly precipitated orformed in a very fine form in the line martensite islands, and there is finally produced a mixed structure of a pure ferrite which is the result of [resulted from the advanced tempering, a tempered martensite and an extremely fine temper-formed r reverted austenite, whereby the toughness of the steel at lower temperature is largely improved, without causing [the 1 reduction of the strength.

However, in the case of the steel containing Ni in a small amount, for instance, less than 4.0 percent and less than 3.5 percent Mn, and the sum of Ni and Mn being less than 4.5 percent, there is found no appearance of the temper-formed or reverted austenite, but instead the precipitation of fine cementite in the abovementioned islands of tempered martensite. The ferrite matrix is also purified, on account of the tempering being well advanced, and becomes a structure [rich in the II which also exhibits great toughness. too II In the following there will be briefly explained the reason [of having limited I for limiting the amounts of the alloy elements to be contained in the steel of the present invention.

C is effective on J for improving the hardenability and further elevating the stability of austenite at low temperature, as it is being absorbed into the austenite precipitated during the temperin treatment. However, if the C content is too [much high, the amount of solid solution carbon in the ferrite matrix will increase, which impairs not only the toughness, but also the weldability of steel. Therefore, the C content [was I is limited to less than 0.2 percent.

Si is an element necessary for steelmaking and is usually contained in an amount of more than 0.05 percent. However, if it exceeds 0.4 percent, there [appears the II is a tendency of the :I that the toughness being deteriorated of the steel may deteriorate.

It is well known that Ni is a useful element for improving the toughness and strength of steel. Further, it serves to stabilize the temper-formed or reverted austenite at low temperature, as it is being absorbed into austenite during the tempering treatment. However, if too much Ni is added [too much [it the steel becomes expensive. Therefore, the upper limit thereof was I is limited to 10.0 percent. On the other hand, if it] the Ni content is less than [.5 percent, the structure which is the object to be attained by the present invention can not be obtained. Therefore, it is necessary to provide a steel which contains Ni in an amount of at least more than l.5 percent and and the preferable range is 4.5 to 9 percent.

Mn [serves to not only improves the hardenability, but also, like Ni and N, stabilizes [a] the very fine austenite precipitated or formed during the tempering treatment and elevated increases the toughness and strength of the ferrite matrix. However, if [it] too much Mn is contained [too much :I in the steel, it stabilizes carbide up to a considerably high temperature. Consequently, the upper limit [thereof was] for Mn is limited to 5.0 percent. Further, Mn is useful as an element which can replace Ni. Therefore, the range of adding 1 for the Mn addition is to be properly decided [depending] in dependence on the Ni content. For instance, if Ni is contained in a range of 4.0 to 7.5 percent, it is [preferably] preferable to [contain add Mn in an amount of more than 0.9 percent. But [,1 in case Cu is added, the lower limit of Mn can be extended to 0.5 percent. On the other hand, in case the Ni content is less than 4.0 percent, there is a possibility of austenite being precipitated orformed, if Mn is added in a relative] large amount, that is, more than l.0 percent. But in [this 1 the latter case, that is, in the case of the Ni content being less than 4.0 percent, there is almost precipitated cementite, if the sum of Ni and Mn is less than 4.5 percent. Therefore, in adding Mn, the above-mentioned conditions must be taken into consideration in order to obtain the desired structure. Further, in case Ni is contained in a large amount, Mn may be added in a small amount. [At least with 0.1% Mn, the I The object of the present invention can be achieved with at least 0.1% Mn. Therefore, the lower limit of Mn [was decided to] is set at 0.1 percent.

Mo has the effect [s] of extending the optimum tempering temperature to the higher side, of refining the distributed states of temper-formed or reverted austenite grains and of promoting the diffusion of Mn, C and N. The addition of M0 is also effective to prevent temper embrittlement. For this purpose Mo must be added in a range of 0.05 to 1.0 percent. The same effeet can also be achieved, [even when a part or the whole of M0 is replaced by W.

Cu may be added, as the occasion demands, in order to improve the corrosion resisting property and the toughness of the steel. Like Ni and Mn, also Cu is thought to be effective [on for stabilizing the temper-formed austenite and for strengthening some of the solid solution ferrite matrix [itself] proper. For this purpose [it] Cu may be added in an amount of less than 2 percent.

Cr is added, as the occasion demands, in order to improve the strength of the steel. Further, it is useful for extending the optimum tempering temperature to the higher side. It is necessary to be added in an amount of 15 0.1 to 1.5 percent.

A1 is necessary to fix nitrogen contained in the steel as an impurity, besides being added as a deoxidizer. A] may be replaced by at least one of other nitride-forming elements such as Be, Nb, V and Ta and the like. If the Al content is, however, too [much 1 high, the impact property of the steel at low temperature of steel] is [deteriorated impaired. Therefore, the Al was content is limited to 0.05 percent in the form of acid-soluble I: one 1 AI.

Further, in the present invention at least one element selected from the group consisting of less than 0.20% V, less than 0.2% Nb, less than 0.1% Zr, less than 0.1% Ti and less than 0.005% B may be present for the purpose of particularly imparting [the] strength to the steel and for promoting the effect of refining crystal grains.

1n the following the examples of the present invention shall be [shown 1 described.

Table 1 HEAT TREATMENTS AND MECHANICAL PROPERTlES OF 6% Ni SERIES STEELS Chemical composition (wt. percent) Plate thick- Steel Sol. Other ness, No. C Si Mn N1 M0 A1 element mm 6A 0.07 0.23 1.70 6.0 0.20 0.15 0.015 Balance Fe. 12 613 0.07 0.23 1.70 6.0 0.20 0.15 0.015 do 12 6C 0.07 0.23 1.70 6.0 0.20 0.15 0.015 d0 12 6D 0.07 0.23 1.70 6.0 0.20 0.15 0.015 do 12 6E 0.07 0.23 1.70 6.0 0.20 0.15 0.0!5 do 12 6F 0.07 0.23 1.70 6.0 0.20 0.15 0.0!5 do 12 60 0.07 0.23 1.70 6.0 0.20 0.15 0.015 do 12 6H 0.07 0.23 1.70 6.0 0.20 0.015 0.015 do 12 61 0.09 0.23 1.10 5.94 0.21 0.01 do 13 61 0.09 0.23 1.10 5.94 0.21 0.01 do 13 6K 0.09 0.23 1.10 5.94 0.21 0.01 do 13 6L 0.09 0.23 1.10 5.94 0.21 0.01 do 13 6M 0.09 0.23 1.10 5.94 0.21 0.01 do 13 6N 0.09 0.23 1.10 5.94 0.21 0.01 do 13 60 0.064 0.11 0.52 6.30 0.22 0.01 Cu 0.84; 13

balance Fe. 6P 0.064 0.11 0.52 6.30 0.22 0.01 do 13 6Q 0.10 0.23 1.70 6.0 0.20 0.01 Nb 0.05; 13

balance Fe. 6R 0.10 0.23 1.70 6.0 0.20 0.01 do 13 Steel Heat treatment No. Q T

6A 700C. X 60 mm. WQ 550C. X 60 mm W0 6B (In 600C. X 60 mm W0 6 C 800C. X 60 mm. AC. 670C. X 60 mm. WQ 500C. X 60 mm WQ 6D do. do 550C. X 60 mm WQ 6 E do. do 600C. X 60 mm WQ 6 F do. do 625C. X 60 mm WQ 6 G do. do 650C. X 60 mm WQ 6 H do. [do A 600C. X 60 mm WQ 6 I do. 670C. X 60 mm. WQ 500C. X 60 mm WQ 6 J do. do 600C. X 60 mm WQ 6 K do. do 625C. X 60 mm WQ 6 L' do. do 500C. X 60 mm WQ 6 M do. do 600C. X 60 mm WQ 6 N' do. do 625C. X 60 mm WQ 6 0 800C. X 60 mm. WQ 670C. X 60 mm. WQ 600C. X 60 mm WQ 6 P' do. do do. 6 Q 800C. X 60 mm. AC. 640C. X 20 [mm] hr. WQ do. 6 R' d0. 625C. X 60 mm WQ Mechanical property Elongation Y R. Total Uniform Steel o'y, kg./ 08, kg./ (try/0B) elongation elongation mvPc No. mm mm percent percent percent kg.mlcm Hv 6 A 25.5 6 B 26.2 6 C 77.9 89.9 0.87. 24.2 12.5 5.75 6 D 71.7 83.6 0.86 28.4 15.1 31.3 6 E 69.9 81.4 0.86 29.8 15.3 29.8 6 F 56.4 87.4 0.64 31.9 20.1 26.0 6 G 57.0 98.5 0.58 26.0 15.3 9.0 6 H 69.6 77.8 0.96 24.9 10.8 13.0 61 70.7 82.9 0.85 24.2 13.8 24.4 6 J 65.6 76.7 0.86 27.4 16.4 27.1 6 K [55.2] 56.2 80.2 0.70 29.9 22.3 23.8

Table l-continued Elongation (.R. Total Uniform Steel o'y, kg/ 0B, kg} (ayfaB) elongation elongation vE Unit No mm mm percent percent percent kgJn/cm Hv 6 L' 79.5 84.2 0.94 20.5 7.6 6.5 6 M 69.4 76.5 0.9l 26.4 14.6 l8.8 6 N 63.3 75.l 0.84 26.0 15.6 26.] 6 O 22.5 236 6 P l0.0 250 6 O 17.5 297 6 R' 12.3 273 Steels treated according to conventional methods Remarks: Heat treatment: Q means a treatment of heating and cooling to and from a temperature above the Ac; transformation point; L means a treatment of heating and cooling to and from a temperature [below the Ac, transformation point] between the Ac, transfilrmulimi point and lhu Ac rrunsformulirm point; T means a tempering treatment at a temperature below the Ac, transformation point, C. is temperature, mm is heating time (minute), W0 is quenching by water, and AC is cooling by air. Mechanical property: rrY is Yield stress, (1B is Tensile strength; Y.R. lrrY/(rB) is Yield ratio; VIE-mew is V-notch Charpy impact test value l96C.', Hv is Vickers hardness. Hr is hearing lime (hour).

EXAMPLE 1 In table 1 there are shown mechanical properties of about 6% Ni-containing steels, subjected to the heat treatment of the present invention. All the samples con sist [ing 1 of steel plates having a prescribed thickness respectively I as are shown in table 1, and have been prepared according to a conventional melting process, ingotting, slabbing and hot-rolling.

Among them, the samples Nos. 6A and 6B are steels which contain 6% Ni and 1.7% Mn. I: and are These samples were obtained by directly subjecting a hotrolled steel to the L-treatrnent (the treatment of heating and cooling to and from a temperature between the Ac, transformation point and the AC3 transformation point). The L-treatment is followed by the T-treatment (tempering at a temperature below the Ac transforma tion point). In the case of the sample No. 6A the tempering temperature was 550C. and in the case of the sample No. 6B 600C. Both samples show an excellent toughness at low temperature. respectively The sample Nos. 6C to 6G are those obtained by subjecting a hot rolled steel plate to the O-treatment at 800C. for 1 hour (the treatment of heating to and cooling from a temperature above the Ac transformation point) followed by the L-treatment at 670C. and thereafter by the T-treatment at 500 to 650C. respectively. The sample No. 6H is a reference steel, which was not subjected to the L-treatment. Concerning mechanical properties of each sample of No. 6C to 6G treated according to the method of the present invention, there are shown excellent values for the toughness at low temperature in spite of a variety of tempering temperature. The reference steel No. 6H shows only a value less than a one half of that of the sample No. 6E of the present invention, even though the sample No. 6H was subjected to the T-treatment under the same conditions as the sample No. 6E. From this comparison the above mentioned advantages will be easily understood. Further, by the application of the method of the present invention, the range of tempering temperature can be largely widened. In FIG. 1 there are shown the relations between the impact value and tempering temperature on steels of the above-mentioned samples Nos. 6C to 66 (shown by a 0 mark) as compared with steels treated according to conventional methods, in which the L-treatment was omitted (shown by a 0 mark). As is evidently seen from this figure, according to the treating method of the present invention,

a tempering treatment for obtaining high impact values can be carried out extending over a far wider range, as above mentioned, whereby [a] the difiiculty in the producing process to the effect that the tempering tre atmcnt must be carried under much narrower conditions, as was heretofore the case, can be eliminated. Moreover, the samples Nos. 6C to 66 show a very low yield ratio respectively, which is particularly manifest in contrast to the sample 6H. It can [be I therefore, be concluded from the foregoing that steels treated by the method of the present invention are superior in the workability I: to] as compared to that of conventional steels.

The samples Nos. 61 to 6N show the results [of] with heat-treated steels having the Mn content reduced to 1.10 percent, [being heat-treated wherein 1 of which the samples Nos. 6] to 6K are those which were treated according to the method of the present invention, while the samples Nos. 6L to 6N are those treated according to a conventional method. The comparison of both groups shows clearly the [quite] substantially same tendency with respect to the toughness at low temperature and the workability of steel as is seen [at] in the comparison of the samples Nos. 6C to 6G with No. 6H. The samples Nos. 60 and 6P are steels, which contain Cu, and show that the high toughness at low temperature can be obtained, even though the Mn content is reduced to about 0.5 percent, if the steel is treated according to the method of the present invention (for instance, No. 60). [At last 1 Finally, the samples Nos. 6Q and 6R are steels, wherein Nb is added. In this case too, an excellent toughness at low temperature can be imparted to steel, if it is heattreated according to the method of the present invention (for instance No. 6Q).

As above mentioned, in the steels subjected to the heat treatment of the present invention the toughness at low temperature is remarkably improved and further the workability is [elevated I increased, as the yield stress is reduced, that is, the yield ratio is lowered, while there is seen no change in the tensile strength. Usually, in the said such steels which contain [5 1 a relatively large amount of Mn, exceeding 1.0 percent and [is further] have, moreover, a high [in the 1 Mo content, cementite remains [till up to the high temperature side [at the time of] during tempering, whereby the toughness is impaired. In this case, if] If the tempering is therefore carried out at a high temperature in order to prevent the above-mentioned pheimenon, the temper-formed austenite becomes unsta e [,1 on cooling, resulting in a deterioration of the ughness. [thereby Howe er, if the steel is treated cording to the method of the present invention, auswherein the Q-treated steel of FIG. 2 was directly subjected to the T-treatment at 600C, resulting in the production of a coarsely precipitated orformed austenite.

nite is precipitated in very fine form and dispersedly EXAMPLE 2 the steel and, in addition thereto, the structure is ry stable, whereby an excellent toughness at low teml 3g?? hq ri j i s rature can be obtained. Further, [as reasons of im- 1 1-60 'l f p oving the toughness of steel there can be enumerated mac lamca lgrc-lpemeslo tame t w 2 following facts that] by the heat treatment of the 19 t e S.amp e 9 1s a i treated zlccordmg a conesent invention, the tempered structure becomes fine g i that S the steel is z i d the ferrite matrix [becomes a] is rendered pure i fi p l P t one as a result of the tempering thereof being well 0 l e presenl mvennon lt ls C ear)! seen t a! t e D d Th sample No. 9B IS has a far lower [in the toughgresse ere are reasons for the improvement of toughness oft/1e Steel-8' ness at low temperature than the sample No. 9A and also in I: the II respect of workability is inferior to the In the following the structures of steels heat-treated latter, as is ShQw" y a higher yield ratio from cording to the present invention shall be explained in FIG 6 I: Showing 1 which Shows the relations between ierence to the attached figures (photograph). The impa value and tempering temperature [on] of the fnples and the treating methods applied correspond Steel o n y t e] ofsample No. 93, as compared the above-mentioned samples N 6E d 6H All with the steel treated according to the method of the ures show microphotographs of 4,500 magnificapresent invention (see black triangle mark), it is seen ti l Fl(} 2 shows a Sample bthat in the case of the reference steel (shown by a :ted to the following heat treatment: a heating to 0 k) the range of P p T'tl'ealmem, that is, the C for 1 hour with a sequent air cooling thererange of tempering temperature capable of obtaining im [,1 I: wherein it It is I: evidently seen high impact value is very narrow, while in the case of 'arly seen that the structure consists of martensite and the steel of the present invention the said range is far rtly bainite. FIG. 3 is a photograph showing a strucwider, indicating that an excellent toughness can be "e obtained by further subjecting the steel having the obtained in the a wide range of tempering temucture as shown in FIG. 2 to the L-treatment of heatperature.

TABLE 2 9%-Ni SERIES STEELS Chemical composition (wt. percent) Heat treatment Plate thickteel Sol. Other ness. los. C Si Mn Ni Mo Al element mm. Q L T A 0.04 0.25 0.43 9.05 001 Balance Fe 13 800 C.X mrn. AC 670 C.X60 mm. WQ 575 C. 60 mm. W B 0.04 0.25 0.43 9.06 0.01 Balance Fe l3 800 C 60 mm. PO

Mechanical property Elongation Total Uniform elongaelonga Steel 0' 03, YR. tion, tion, lQlW'. No kg/mm ltgJmm (o'Y/u'B) percent percent kgml'rm in Table I Remarks: PQ is pressure quench lher signs are of the same meaning a: in Table I to 560C. for l hour and a sequent water cooling. am this photograph it is clear that as a result of the :reatment there is produced a structure, wherein it martensite transformed from precipitated or retedly formed austenite enriched in C, Ni. Mn and N d precipitated finely in an insular form at an old marisite subgrain boundary, is equilibrated with ferrite, IS forming a mixed structure of them. The most imrtant feature of the present invention liesjust in pro- :ing the above-mentioned structure. When the steel ring the structure as above mentioned is further subted to the T-treatment of heating the steel to 600C. l hour with a sequent water cooling, there IS disrsedly precipitated or formed an ultrafine austenite in tempered martensite, as shown in FIG. 4. This will be more clearly observed, when compared .h the structure of the reference sample shown in 3. 5. That is, FIG. 5 thus shows a structure of the el treated according to a conventional method,

The structures of steels subjected to various heattreating methods in the present example are I: as I shown by microphotograph of 4,500 magnifications in FIGS. 7 to 10. [respectively 1. FIG. 7 is a microphotograph showing the structure obtained by the Q- treatment. From this photograph it is seen that the structure [of] in this case consists of a mixed structure of martensite and partly bainite. FIG. 8 shows the structure obtained by subjecting the steel having the structure shown in FIG. 7 to the L-treatment wherein II. It is clearly seen that fine martensite in the insular form is precipitated orformed in the ferrite matrix. When further subjecting the sample of FIG. 8 to the T-treatment, there can be obtained a structure as shown by the photograph in FIG. 9, wherein the tempered martensite is formed in the ferrite matrix, where the tempering made progress, in said tempered martensite ultra fine austenite being precipitated orform ed. For the purpose of comparison there is shown in FIG.

the final structure of sample No. 9B, which was not subjected to the L-treatment. From this figure it is quite evident that the temper formed or reverted austenite has a coarse structure, as compared with the structure shown in FIG. 9.

TABLE 3 3%-Ni SERIES STEELS Chemical composition (wt. percent) Plate thick- Heat treatment Steel Sol. Other ness,

No. C Si Mn Ni Mo Al element mm. 0 L T 3 A 0.10 0.24 0.69 3.03 0.36 Balance 11 900C.X60 mm. WQ 720C.X00 mm. WQ 500C.X60 mm. WQ

3 B 0.10 0.24 0.69 3.03 0.36 do. 11 do. do. 550C.X60 mm. WQ

3 C 0.10 0.24 0.69 3.03 0.36 do. 11 do. do. 600C. 60 mm. WQ

3 D 0.10 0.24 0.69 3.03 0.36 do. 11 900C.X00 mm. W Q 500C. 60 mm. WQ

3 E 0.10 0,24 0.69 3.03 0.36 do. 1 1 do. 600C.X60 mm. WQ

3 F 0.045 0.21 4.45 2.95 0.21 0.015 do. 1 1 800C. 00 mrn. WQ 670C. 60 mm. WQ do.

3 G 0.045 0.21 4.45 2.95 0.21 0.015 do. 11 do. 700C.X60 mm. WQ do.

3 H 0.045 0.21 4.45 2.95 0.21 0.015 do. 11 do.

Mechanical property Steel Vlrs v No. (C.) 3 A 190 205 3 B 175 195 3 C 165 181 3 D' l60 249 3 E 150 207 3 F 11.0 253 3 G 15.6 260 3 H' 8.1 235 See Table l Remarks: vTrs C.) is V-notch Charpy 50% ductile transition temperature. Other signs are of the same meaning as FIG. 1.

EXAMPLE 3 In Table 3 there are shown mechanical properties, particularly the toughness and strength, of about 3.0% Ni-containing hot-rolled steel plates manufactured by a conventional method, as subjected to the Q-treatment and the T-treatment with or without the L-treatment.

The samples Nos. 3A and 3C and Nos. 3F and 36 are the steels treated according to the method of the present invention, including the L-treatment, while the samples Nos. 3D, 3E and 311 are those treated I: to J by a conventional method, wherein the L-treatment is not included. From the comparison of both groups it is clear that the steels of the present invention are far superior in the toughness [to I as compared to that of conventional steels. [That is, 1 Thus, in the steels treated according to the method of the present invention there is produced a very fine tempered structure, wherein cementite is finely precipitated in islands of the tempered martensite. Further, as the tempering of the ferrite matrix [is made makes progress and there are 1 is produced pure ferrite of fine grains which does not contain cementite, a structure having an excellent toughness can be obtained. FlG. 11 is a microphotograph of 4,500 magnifications of the final structure of the sam le No. 3A, from which the above-mentioned fact is Il manifested to be true I apparent. FIG. 12 is also a microphotograph of the final structure of the sample No. 3D which was not treated according to the method of the present invention, showing that a coarse cementite is being extensively precipitated, which establishe [d] s that the steel treated according to the method of the present invention is superior to the conventional steel.

As is above explained, [the steel, heat-treated according to the method of the present invention, has an excellent toughness at low temperature without proved. However, the present invention is not limited to the examples above-mentioned, but may be modified variously within the scope of the objects of the present invention.

What is claimed is:

1. A heattreating method for producing a steel having an excellent toughness at an extremely low temperature and [a] high strength and good workability, from a steel consisting essentially of less than 0.20 wt.% C [.1 0.05 to 0.40 wt.% Si, 0.10 to 5.0 wt.% Mn, 1.50 to 10.0 wt.% Ni as basic alloying component, with the balance being Fe and the usual impurities, comprising subjecting a hot-rolled steel plate having the said composition to a heat-treatment comprising heating to a temperature between the Ac transformation point and the Ac, transformation point and a subsequent cooling from the said temperature at least at an air cooling rate and thereupon tempering the steel plate at a temperature below the Ac transformation point [.1 but sufficiently high so as to cause precipitation or formation of fine austenite.

2. A method according to claim 1, wherein the heattreatment comprising the heating of the hot-rolled steel plate to a temperature between the Ac transformation point and the Ac transformation point and the subsequent cooling from the said temperature is carried out repeatedly more than one time.

3. A method according to claim 2, wherein prior to said heat-treatment, said hot-rolled steel plate is subjected to a solid solution treatment.

4. A heat-treating method for producing a steel having an excellent toughness at an extremely low temperature and a high strength and good workability from a steel consisting essentially ofless than 0.20 wt.% 0, 0.05 to 0.40 wt.% Si, 0.10 to 5.0 wt.% Mn, [m 1 1.50 to 10.0 wt.% Ni as basic alloying components with the balance being Fe and the usual impurities, comprisng heating a hot-rolled steel plate having the said comiosition to a temperature below the austenite grain :oarsing temperature but above the AC3 transformation )Ollll and subsequently cooling the steel plate from the aid temperature at least at an air-cooling rate, then ubjecting the steel plate to a heat-treatment comprisng heating the steel plate to a temperature between the \c, transformation point and the Ac; transformation toll and subsequently cooling said steel plate from the aid temperature at least at an air cooling rate and hereupon tempering the steel plate at a temperature relow [J the AC1 transformation point but sujficienrly u'gh so as to cause precipitation orformation affine ausenite.

5. A heat-treating method according to claim 4, vherein the heat-treatment comprising the heating of he steel plate to a temperature between the AC1 transormation point and the Ac transformation point and he subsequent cooling from the said temperature at L 6. A heat-treating method according to claim 4, wherein said hot-rolled steel plate is subjected to a solid solution treatment prior to heating the steel plate to a temperature below the austenite grain coarsing temperature but above the Ac transformation point and subsequently cooling the steel plate from the said temperature.

7. A method as claimed in claim 1 wherein said steel further comprises at least one element selected from the group consisting of 0.05 to 1.0 wt.% M0, 0.1 to 2.0 wt.% Cu, 0.1 to 1.50 wt.% Cr, less than l.0 wt.% Nb, less than l.0 wt.% V and less than 0.05 wt.% sol. Ao Al, and the rest being Fe and unavoidable impurities.

8. A method as claimed in claim 4, wherein said steel further comprises at least one element selected from the group consisting of 0.05 to 1.0 wt.% M0, 0.1 to 2.0 wt.% Cu, 0.1 to 1.50 wt.% Cr, less than 1.0 wt.% Nb, less than 1.0 wt.% V and less than 0.05 wt.% sol. Al and the rest being Fe and unavoidable impurities. 

1. A HEAT-TREATING METHOD FOR PRODUCING A STEEL HAVING AN EXCELLENT TOUGHNESS AT AN EXTREMELY LOW TEMPERATURE AND A HIGH STRENGTH AND GOOD WORKABILITY FROM A STEEL CONSISTING ESSENTIALLY OF LESS THAN 0.20 WT. % C (.), 0.05 TO 0.40 WT. % SI, 0.10 TO 5.0 WT. % MN, 1.50 TO 10.0 WT.% NI AS A BASIC ALLOYING COMPONENT, WITH THE BALANCE BEING FE AND THE USUAL IMPURITIES, COMPRISING SUBJECTING A HOT-ROLLED STEEL PLATE HAVING THE SAID COMPOSITION TO A HEAT-TREATMENT COMPRISING HEATING TO A TEMPERATURE BETWEEN AC1 TRANSFORMATION POINT AND THE AC3 TRANSFORMATION POINT AND A SUBSEQUENT COOLING FROM THE SAID TEMPERATURE AT LEAST AT AN AIR COOLING RATE AND THEREUPON TEMPERING THE STEEL PLATE AT A TEMPERATURE BELOW THE AC1 TRANSFORMATION POINT (.) BUT SUFFICIENTLY HIGH SO AS TO CAUSE PRECIPITATION OR FORMATION OF FINE AUSTENITE.
 2. A method according to claim 1, wherein the heat-treatment comprising the heating of the hot-rolled steel plate to a temperature between the Ac1 transformation point and the Ac3 transformation point and the subsequent cooling from the said temperature is carried out (repeatedly) more than one time.
 3. A method according to claim 2, wherein prior to said heat-treatment, said hot-rolled steel plate is subjected to a solid solution treatment.
 4. A heat-treating method for producing a steel having an excellent toughness at an extremely low temperature and (a) high strength and good workability from a steel consisting essentially of less than 0.20 wt.% C, 0.05 to 0.40 wt.% Si, 0.10 to 5.0 wt.% Mn, (to) 1.50 to 10.0 wt.% Ni as basic alloying components with the balance being Fe and the usual impurities , comprising heating a hot-rolled steel plate having the said composition to a temperature below the austenite grain coarsing temperature but above the Ac3 transformation point and subsequently cooling the steel plate from the said temperature at least at an air-cooling rate, then subjecting the steel plate to a heat-treatment comprising heating the steel plate to a temperature between the Ac1 transformation point and the Ac3 transformation point and subsequently cooling said steel plate from the said temperature at least at an air cooling rate and thereupon tempering the steel plate at a temperature below(. the Ac1 transformation point but sufficiently high so as to cause precipitation or formation of fine austenite.
 5. A heat-treating method according to claim 4, wherein the heat-treatment comprising the heating of the steel Plate to a temperature between the Ac1 transformation point and the Ac3 transformation point and the subsequent cooling from the said temperature at least at an air cooling rate is carried out (repeatedly) more than one time.
 6. A heat-treating method according to claim 4, wherein said hot-rolled steel plate is subjected to a solid solution treatment prior to heating the steel plate to a temperature below the austenite grain coarsing temperature but above the Ac3 transformation point and subsequently cooling the steel plate from the said temperature.
 7. A method as claimed in claim 1 wherein said steel further comprises at least one element selected from the group consisting of 0.05 to 1.0 wt.% Mo, 0.1 to 2.0 wt.% Cu, 0.1 to 1.50 wt.% Cr, less than 1.0 wt.% Nb, less than 1.0 wt.% V and less than 0.05 wt.% sol. (Ao) Al , and the rest being Fe and unavoidable impurities.
 8. A method as claimed in claim 4, wherein said steel further comprises at least one element selected from the group consisting of 0.05 to 1.0 wt.% Mo, 0.1 to 2.0 wt.% Cu, 0.1 to 1.50 wt.% Cr, less than 1.0 wt.% Nb, less than 1.0 wt.% V and less than 0.05 wt.% sol. Al and the rest being Fe and unavoidable impurities. 